The process of tyrosinase formation and melanization of melanosomes will be investigated in cell cultures of retinal pigment epithelial cells of 8 day-old chicken embryos. In particular, the synthesis of DNA and various RNA's will be studied. The epidermal melanocyte system of mammals will be also investigated, with particular reference to the surface properties of the epidermal melanocytes as studied in the scanning electron microscope and by histochemical methods in cell cultures of melanocytes and keratinocytes and in vivo.